The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Low density plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent to the target impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate to position vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered in a low density plasma often has an ionization degree of less than 1% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance-matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
In a number of deposition chambers such as a physical vapor deposition chamber, the chamber walls are often formed of a conductive metal such as stainless steel. Because of the conductivity of the chamber walls, it is often necessary to place the antenna coils or electrodes within the chamber itself because the conducting chamber walls would block or substantially attenuate the electromagnetic energy radiating from the antenna. As a result, the coil and its supporting structures are directly exposed to the deposition flux and energetic plasma particles. This is a potential source of contamination of the film deposited on the wafer, and is undesirable.
To protect the coils, shields made from nonconducting materials, such as ceramics, can be placed in front of the coil. However, many deposition processes involve deposition of conductive materials such as aluminum on the electronic device being fabricated. Because the conductive material will coat the ceramic shield, it will soon become conducting, thus again substantially attenuating penetration of electromagnetic radiation into the plasma. Consequently, it is preferred to place a shield wall behind the coil to protect the interior of the deposition chamber from the deposition material. However, the problem of particulate matter remains for sputtering chambers of this design.
It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter-depositing a layer which obviate, for practical purposes, the above-mentioned limitations.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention, a plasma-generating apparatus which comprises at least one electron source which injects energetic electrons into a semiconductor fabrication chamber to initiate and sustain a relatively high density plasma at extremely low pressures. In addition to ionizing atoms of the extremely low pressure precursor gas, such as an argon gas at 100 microTorr, for example, the energetic electrons are also believed to collide with target material atoms sputtered from a target positioned above a substrate, thereby ionizing the target material atoms and losing energy as a result of the collisions. As a consequence, coils or other structures for inductively coupling RF power to sustain a plasma can be eliminated.
Preferably, the electrons are injected by electron guns positioned to inject the electrons substantially tangentially to the walls of a chamber shield surrounding the high density plasma into a confining magnetic field. The magnetic field is preferably oriented generally parallel to a central axis of the semiconductor fabrication chamber and substantially perpendicular to the surface of the substrate. As the injected electrons lose energy, colliding with and ionizing the target material atoms, and atoms of the extremely low pressure precursor gas, the electrons spiral inward toward a central region of the semiconductor fabrication chamber surrounding the central axis, forming an electron cloud in the central region. An arrangement of electromagnets may be positioned adjacent the walls of the chamber shield surrounding the high density plasma to generate the confining magnetic field. It is believed that the configuration of confining magnetic fields also keeps electrons from colliding with the walls of the chamber shield surrounding the high density plasma.